The present invention relates to an optical system for tracking an information recording medium such as an optical disk with multiple beams for information recording and reproduction, or for tracking a medium for information recording, reproduction and erasure so as to record information in and reproduce it from the information recording medium or to record information in and reproduce and erase it therefrom. More particularly, it relates to an optical system for detecting the focusing state of an objective lens for focusing the multiple beams.
Various types of optical heads have been recently developed and made commercially available to record information in or reproduce it from an optical disk. In such an optical head, a laser beam which is intensity-modulated by information to be recorded is focused by an objective lens onto a recording surface of a rotating optical disk, and the state of regions on the recording surface is continuously changed. For example, apertures such as pits are formed in the recording surface, or optical characteristics such as a refractive index and a reflectance are changed to record information in the recording surface of the optical head. A reproduction laser beam is focused by the objective lens onto such a recording surface, and the reproduction laser beam is intensity-modulated by the changed states of the regions. The reproduction laser beam is converted by a photodetector to electrical signals, thereby reading out the information.
Another conventional optical disk is also known which comprises a tracking guide for specifying a region or area in which information is to be recorded. In this type of optical head, the reproduction beam is focused in both the reproduction and recording modes. In the recording mode, the reproduction beam is used to check whether or not information is properly recorded in the tracking guide. In such an optical head, since the reproduction light beam traces the region after the recording beam has tracked it, the recording beam inevitably extends outside the recording region. Thus, a region outside the recording region is damaged by the recording beam. In addition, if there is a defective portion in the tracking guide, since it cannot be detected, information is written in the defective portion. In this case, information cannot be recorded or erroneous information is recorded in the defective portion.
In order to solve this problem, a prebeam is converged on the optical disk in addition to the recording and reproduction beams. An optical head of this type is described in U.S. Ser. No. 673,764, abandoned, filed on Nov. 21, 1984, abandoned, and in a corresponding EPC Application No. 84114044.5 filed on Nov. 20, 1984 wherein the region tracked by the prebeam is tracked again by the recording and reproduction light beams. In such an optical head, a plurality of light beams are incident on the optical disk at slightly different incident angles, and a plurality of light beams reflected by a light reflecting surface of the optical disk are guided to a tracking guide and focusing state detecting optical system. In this system, a plurality of beams are separated to cause a single light beam to be incident on a photo-sensitive region of a photodetector. In U.S. Ser. No. 673,764 filed on Nov. 21, 1984, abandoned, and in the corresponding EPC Application No. 84114044.5 filed on Nov. 20, 1984, a single light beam is separated by a knife edge or light shielding plate from a plurality of light beams to detect the focusing state of an objective lens and a tracking guide. However, in order to properly detect the focusing state by using the knife edge, the light receiving surface of the photodetector must be determined by an optical system arranged between the optical disk and the light receiving surface of the photodetector, and in the just-in-focus state, a beam spot image formed on the optical disk must be properly located on the focusing surface, i,e., the focal plane of the projection lens.